Diagnosing of urological problems is facilitated by the analysis of information relating to the natural voiding of urine by the patient. Various instruments have been used in the past to measure such characteristics of the urine stream as total volume and average flow rate but no satisfactory instrument has been available for quickly and accurately indicating the instantaneous flow rate of the stream. However, such information and in particular the maximum flow rate of the urine stream is indicative of certain urological conditions. It is also important that the instrument provide repeatable measurements so that the progress of the patient can be monitored from time to time.